This invention relates generally to exhaust gas sensors, and specifically to oxygen sensors.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and air to fuel ratio of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:       E    =                  (                  RT                      4            ⁢            F                          )            ⁢              xe2x80x83            ⁢              ln        ⁡                  (                                    P                              O                2                            ref                                      P                              O                2                                              )                                                  where          ⁢                      :                                                        E          =                      electromotive            ⁢                          xe2x80x83                        ⁢            force                                                        R          =                      universal            ⁢                          xe2x80x83                        ⁢            gas            ⁢                          xe2x80x83                        ⁢            constant                                                        F          =                      Faraday            ⁢                          xe2x80x83                        ⁢            constant                                                        T          =                      absolute            ⁢                          xe2x80x83                        ⁢            temperature            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                        ref                    =                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            reference            ⁢                          xe2x80x83                        ⁢            gas                                                                    P                          O              2                                      xe2x80x83                                =                      oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            exhaust            ⁢                          xe2x80x83                        ⁢            gas                              
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture.
Increased demand for improved fuel economy and emissions control has necessitated the development of oxygen sensors capable of quantifying the exhaust oxygen partial pressure over a wide range of air fuel mixtures in both fuel-rich and fuel-lean conditions. As is taught by U.S. Pat. No. 4,863,584 to Kojima et al., U.S. Pat. No. 4,839,018 to Yamada et al., U.S. Pat. No. 4,570,479 to Sakurai et al., and U.S. Pat. No. 4,272,329 to Hetrick et al., an oxygen sensor which operates in a diffusion limited current mode produces a proportional output which provides a sufficient resolution to determine the air-to-fuel ratio under fuel-rich or fuel-lean conditions. Generally, diffusion limited current oxygen sensors have a pumping cell and a reference cell with a known internal or external oxygen partial pressure reference. A constant electromotive force, typically corresponding to the stoichiometric electromotive force, is maintained across the reference cell by pumping oxygen through the pumping cell. The magnitude and polarity of the resulting diffusion limited current is indicative of the exhaust oxygen partial pressure and, therefore, is a measure of air-to-fuel ratio.
As is taught by U.S. Pat. No. 4,450,065, wide range oxygen sensors commonly employ an aperture with a cross-sectional area to length ratio sufficiently small to limit exhaust gas diffusion. In this sensor, a gap between the pumping and reference cells forms such an aperture and limits diffusion of exhaust gas to a common environment between the two cells. This common environment, or diffusion chamber is required in an aperture construction for adequate mixing of the diffused exhaust gas; however, it tends to slow the frequency response of the sensor operation. Additionally, although the two electrodes adjacent to the diffusion chamber can be shorted together to eliminate one lead, four separate electrodes are required in this construction.
Commonly assigned U.S. Pat. No. 5,360,528 to Oh et al., teaches a wide range oxygen sensor having improved mass production capabilities. This wide range oxygen sensor employs a porous layer, formed by plasma spray deposition, to limit oxygen diffusion in lieu of the diffusion limiting aperture. This wide range oxygen sensor has a planar structure with a single solid electrolyte layer shared by electrochemical storage, pumping, and reference cells. The electrochemical pumping cell has a diffusion layer formed from a porous ceramic to permit diffusion of oxygen molecules through this layer.
Planar exhaust sensor elements comprising a plurality of rectangular layers are known to reach operating temperature more rapidly than conical sensors. Planar exhaust sensors have been reduced in size in order to reach operating temperature even more rapidly, but size reduction requires increased complexity for the electrical interconnection to the sensor element. Additionally, leads to the heater element disposed within the sensor element must be reduced in size as well, which leads to greater electrical resistance in the heater element leads, and a commensurate loss of energy.
Also, linear oxygen sensors can have two to five more leads than conventional stoichiometric sensor elements. The extra leads require connections that must be secured to the exterior of the connection end of the sensor element. As sensor size is decreased to gain a performance advantage, the area for connecting the extra leads to external circuits is reduced, and ensuring secure connections becomes more difficult.
What is needed in the art is a sensor element that reaches operating temperature more rapidly than conventional sensors, without increasing energy losses, and without significantly reducing the sensor area in which leads are connected to external circuits.
Herein is described an electrochemical cell, an exhaust gas sensor element, and a method for using the sensor element. The electrochemical cell comprises: a substrate layer with a terminal end and a sensor end, wherein the sensor end is narrower than the terminal end; an electrolyte disposed in the sensor end; an outer electrode disposed in intimate contact with one side of the electrolyte; and, an inner electrode disposed in intimate contact with another side of the electrolyte, opposite the outer electrode.
The sensor element comprises: a plurality of layers comprising a sensor end and a terminal end opposite said sensor end, wherein said layers are disposed in physical contact in a stack, and the width of said sensor end is smaller than the terminal end width. An electrochemical cell is disposed in said sensor end of said layers; and a plurality of electrode leads disposed in electrical contact with the cell, extending from the cell to the terminal end.
The method of using this sensor element comprises: exposing the sensor element to the exhaust gas; diffusing molecular oxygen in the exhaust gas through a gas diffuser to the cell; ionizing the molecular oxygen at an inner electrode of the electrochemical cell; applying a potential between the inner electrode and an outer electrode of the electrochemical cell; and measuring a current produced by the potential.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.